The present invention relates to a magneto-optical recording device for performing write-read with respect to a magneto-optical film having an easy axis of magnetization in a direction perpendicular to the film face of a recording medium by using a laser beam, and in particular to a magnetic-fieldless magneto-optical recording device capable of over-writing information without using a special reversible magnetic field applying means and a method of performing multi-state recording or half-tone recording by using difference in recorded mark size.
An example of prior art of magneto-optical recording for performing write-read with respect to a magneto-optical recording film having an easy axis of magnetization in a direction perpendicular to the film face of the recording film itself by using a laser beam has configuration as shown in FIG. 3, for example.
First of all, in write operation, a beam emitted from a laser 13 shown in FIG. 3 is collimated by a lens 9 into a parallel beam. The beam passed through a beam splitter 10 is focused by another lens 9 to form a light spot on a magneto-optical recording medium 8. The intensity of this beam is modulated according to the recording information. The beam absorbed at the light spot raises the temperature of the magneto-optical recording medium near the Curie temperature. At high temperatures, the coercive force of the magneto-optical recording medium is extremely small. By a bias magnetic field applied onto the magneto-optical recording medium by a magnetic field applying means 15, magnetization of a portion of the magneto-optical recording medium heated by the laser beam is reversed, a reversed domain being thus formed. When information written as this reversed domain is to be read, the same laser beam as that used in the write operation with intensity weakened is applied onto the magneto-optical recording medium. The resultant reflected beam is led to an optical system for reproducing by the beam splitter 10. Rotation of polarization caused by the Kerr effect is then detected. For rewritting this information written once, the above described laser beam is continuously radiated and the polarity of the magnetic field applied by the magnetic field applying means is made opposite to that in the write operation, the recorded reversed domain being thus erased. Thereafter, write operation is performed in the same way as the foregoing description. This method is described in JP-B-57-34588 (Published on Jul. 23, 1982, corresponding to U.S. Ser. No. 284,513 filed on Aug. 29, 1972), for example.
In the above described example of the prior art, however, the procedure of erasing information once and then writing information must be executed in rewritting operation. Therefore, several over-writing methods allowing direct over-writing without the necessity of erasing have already been proposed.
FIG. 4 shows an example of configuration for overwriting. With reference to FIG. 4, a beam emitted from a laser 13 is collimated into a parallel beam by a lens 9 in the same way as the above described example of FIG. 3. The beam passed through a beam splitter 10 is focused by another lens 9 to form a light spot on a magneto-optical recording medium 8. The laser beam is continuously radiated onto a magneto-optical recording medium 8. A magnetic field modulated at high speed according to recording information is applied by a flying magnetic head 16 to a portion of the magneto-optical recording medium 8, which has been so heated by the laser beam as to be nearly at the Curie temperature and have a reduced coercive force. In this way, magnetization of the magneto-optical recording medium 8 is directed in the direction of the applied magnetic field, a recorded domain being thus formed. At this time, high-speed modulation with a recording frequency of 10 to 20 MHz is possible. Since the laser beam is continuously radiated, a recorded domain is formed irrespective of information recorded before, high-speed overwriting being realized. At this time, this flying magnetic head 16 is stably held at a point where buoyancy due to an air current caused by the rotation of the magneto-optical recording medium 8 balances the force of a supporting spring 17. The spacing between this flying magnetic head 16 and the magneto-optical recording medium 8 is 5 to 10 .mu.m. This method has been proposed in JP-A-63-217548, for example.
In the above described example of the prior art, means for applying a reversible bias magnetic field to the radiation position of the optical beam must be provided, resulting in complicated configuration of the apparatus. It thus becomes difficult to reduce the size and cost of the apparatus.
Further, in the above described example of the prior art, the width of the recorded domain varies according to the environment, wherein the apparatus is used, such as temperature. This results in a problem that residue of information recorded before is caused.
Further, in the above described example of the prior art, the recorded domain does not take the shape of a circle but takes the shape of chevron. Therefore, the shape of the domain is not necessarily uniform. As a result, noises in recording operation are increased, high density recording being difficult.
Further, in the above described example of the prior art, the write rate of recording information (i.e., the data transfer rate) depends upon the reversing rate of the applied magnetic field, high-speed recording being not easy.
Further, in the prior art, multi-state or analog high density recording is not easy because the apparatus becomes complicated and residue tends to be caused.